1. Field of the Invention
The present invention relates to a vibration control apparatus that is suitable for application to a wind turbine generator.
2. Description of Related Art
Today, wind turbine generators are being made larger and larger in order to improve their power generating efficiency and to increase the amount of generated power, and ones having a hub height (height from ground to center of hub) exceeding 100 m have been designed for practical use.
However, with those that have a hub height exceeding 100 m, the first-mode natural frequency of the tower matches the resonance range associated with the rotation of the rotor head and the wind-turbine rotary blades, and thus, the fatigue load might significantly increase. Therefore, the thickness of the tower needs to be increased so as to reduce the fatigue load applied to the tower, and thus, there is a problem in that the weight of the tower significantly increases.
Accordingly, to avoid such problems, by using, for example, a vibration control apparatus disclosed in Japanese Unexamined Patent Application, Publication No. 2008-31735, the response can be reduced even when the first-mode natural frequency of a tower matches the resonance range associated with the rotation of the rotor head and wind-turbine rotary blades.
However, when the vibration control apparatus disclosed in Japanese Unexamined Patent Application, Publication No. 2008-31735 is applied to a wind turbine generator, one end of a damper (attenuation device) is secured to an inner wall surface of the tower. Therefore, the inner wall surface of the tower has to be reinforced, and thus, there is a problem in that the weight of the tower increases.
Furthermore, to apply a vibration control apparatus to a wind turbine generator, the structure should not interfere with the beauty of the environment, and it needs to be installed in a small space in the tower; therefore, the size of the vibration control apparatus needs to be minimized.
Furthermore, during assembly of the wind turbine generator, the natural frequency of the tower changes (becomes smaller) in each of the following stages: a stage in which only the tower is completed; a stage in which a nacelle is mounted at the upper end of the tower; and a stage in which a rotor head and wind-turbine rotary blades are attached to the nacelle, completing the entire wind turbine generator. Therefore, it is preferable that the natural frequency of the vibration control apparatus can be variable accordingly.